Photo-lithography, using either positive or negative emulsions, is used to define images in a mask layer of material deposited on a glass plate or substrate. This technique is widely used in the semiconductor industry to form a wide range of structures in integrated circuit chips. As the size of the production runs of such integrated circuit chips increased and as automatic mask handling equipment came into wide use, it became necessary to provide the images in a mask layer of hard, abrasion-resistant material, such as a metal (typically a deposit of chrome), deposited on the surface of the substrate. The material used as the mask layer, must be imagable with a high resolution such that the desired image can be readily and easily defined therein.
Although such metallic coated masks have a long production life, they require the use of a transfer agent, e.g., a photoresist layer, in which the desired image is first formed and from which the image can be transferred into the mask layer. As the chip feature size decreases, it becomes increasingly difficult to provide perfect masks. This occurs because very small particles of foreign material, bubbles in the photoresist, or other anomalies approach the size of the image to be formed in the transfer layer. These defects prevent proper resist exposure or alter the effects of the developer on the exposed photoresist. This results in unwanted patterns being created in the exposed and developed photoresist. In the prior art processes, such unwanted patterns are transferred into the mask layer. These defects are especially troublesome in phase shift masks.
To correct for such errors in the mask layer, elaborate and expensive mask inspections and processes for the repair of the defined images in the metallic mask layer have been created. Such repair procedures require modification of either the surface of the mask layer or the surface of the substrate. The prior art repair techniques require treatments, such as ion beam sputtering or laser beam ablation to remove unwanted mask layer material from the surface of the substrate, in areas designed to be fully transmissive, or the deposition of additional, suitably opaque, material in the those areas designed to be opaque. These repair techniques typically yield either unacceptable results, or introduce undesirable phase or transmission defects in the final mask.
Accordingly, there now exists a need for an improved mask making process that avoids the problems of the prior art and that substantially eliminates the need for repair processes that tend to introduce undesirable defects in the final mask.